Natural gas produced from gas fields or oil fields includes many impurities such as CO2. After such impurities such as CO2 have been removed from the natural gas, the resultant gas is shipped as product gas. In a known CO2 removal method, CO2 is absorbed and removed from the natural gas by use of an amine absorbing liquid as disclosed, for example, in JP 2005-296817 A. This method requires regeneration of the absorbing liquid that has absorbed much CO2 by heating the absorbing liquid to release CO2. Thus, when used, in particular, in processing natural gas with a high concentration of CO2, the method requires a large amount of energy and thus incurs huge operating costs, which is problematic.
In another known CO2 removal method, CO2 is removed using a gas separation membrane with high CO2 selectivity. In this connection, JP 2009-113994 A discloses a related method, although its subject gas is exhaust gas from a power plant. In the method, the exhaust gas is pressurized by a compressor of a gas turbine, and CO2 is then removed therefrom by use of a gas separation membrane. Such a method using a gas separation membrane is superior to the amine absorption method in terms of operating cost.